Originally Posted by MyDogSkip
. . . my CPU seems to be very slightly concave. . . .
Your heatsink is resting on the edges of the ihs (integrated heat spreader). The cpu itself is in the center of the ihs. You need firm contact with the cpu, making the ihs a sandwich between the cpu and the 212+ contact plate.
Probably the best way to improve your temps at this point is to lap your cpu. BTW -- never lap an HDT heatsink like the 212+. HDT heatsinks already have their heapipes partially ground down to make them flat.
Another thing: the reason you get lower temps is that Prime95 does not stress the cpu as much as Linpack, the engine behind IBT. Prime95 is a better test of OC stability, but Linpack was designed by Intel to put the max thermal stress on cpu's, and that's just what I found. I use OCCT/Linpack to thermally stress my cpu for my various fan tests. You might get a somewhat lower average temp because the app rests between bouts, but its plateaus are the hottest.